1. Field of the Invention
The present invention relates to an optical device, and more particularly, it relates to an optical device provided with a multi-mode waveguide path.
2. Description of the Related Art
There has conventionally been provided an optical coupler in the field of optical communication. An optical coupler refers to a device for branching one signal into an N (N is 2 or a larger integer) number of ports for output. Such an optical coupler may be equipped with a multi-mode waveguide path, to provide a certain type of optical coupler (hereinafter called multi-mode waveguide type optical coupler). An example of a configuration of this multi-mode type optical coupler is disclosed in the literature (IEEE Laser and Electro-optics Society (LEOS), 1983, pp. 193-194), which is hereinafter referred to as a first coupler.
The following will describe the configuration of a prior-art first coupler 100 with reference to FIG. 8. FIG. 8 is a plan view of the first coupler 100 as viewed from the upper side of the main surface of a clad layer. Actually, however, since a waveguide path (i.e., core) 102 is covered by a clad layer 104 and a substrate (not shown in FIG. 8), the waveguide path 102 cannot directly be seen from the upper side of the main surface of the clad layer 104, but to spotlight the shape of the waveguide path 102, FIG. 8 shows the shape of the waveguide path 102 (hatched portion) on this main surface. This waveguide path 102 comprises two input waveguide paths 106A and 106B, a multi-mode waveguide path 108, and three output waveguide paths 110A through 110C. In this case, however, the waveguide path 102 is supposed to have a uniform (constant) refractive index. Furthermore, the input waveguide paths 106A and 106B and the output waveguide paths 110A-110C are each supposed to be a single-mode waveguide path, although they may be a multi-mode one. The multi-mode waveguide path 108 has a rectangular shape as viewed from the upper side of the main surface of the clad layer 104. The major axis direction of this rectangle agrees with a direction in which the light propagate.
Next, the propagation form of an optical signal in the first coupler 100 is described below. This example is explained with reference to a case where a predetermined single-wavelength optical signal is input from outside to the input waveguide path 106A, i.e., the input waveguide path 106B is not used. This optical signal propagates through the input waveguide path in the single mode and then enters the multi-mode waveguide path 108. This optical signal enters the multi-mode. This optical signal in the multi-mode propagates through this multi-mode waveguide path 108, to subsequently enter all (or either one) of the output waveguide paths 110A-110C. The optical signals thus propagating through the respective output waveguide paths 110A-110C enter the single mode again. Note here that the power ratio (hereinafter called coupling ratio) of these optical signals coupled to the output waveguide paths 110A-110C is determined beforehand based on the configuration of this first coupler 100.
According to the configuration of the first coupler 100, however, since the refractive index of the multi-mode waveguide path 108 is constant, the coupling ratio is limited within a certain range and therefore cannot always be set at a desired value, which leads to a problem.
According to the configuration of the first coupler 100, however, since the refractive index of the multi-mode waveguide path 108 is constant, the coupling ratio is limited within a certain range and so cannot always be set at a desired value, which leads to a problem.
To guard against this, there is provided such an optical coupler that can change its refractive index in the multi-mode waveguide path to control the mode-field distribution of an optical signal in the multi-mode waveguide path, thus obtaining a desired coupling ratio. FIG. 9 is a plan view as viewed from the upper side of the main surface of a clad layer of a second prior-art coupler. A multi-mode waveguide path 202 comprises a high refractive index region 202A, a middle refractive index region 202B, and a low refractive index region 202C. These regions 202A-202C are partitioned off from each other by a boundary line running parallel to the relevant major axis. The high refractive index region 202A lies at the middle portion of the multi-mode waveguide path 202 and in the middle refractive index region 202B. The middle refractive index region 202B, on the other hand, is enclosed by the low refractive index 202C. FIG. 10 is a graph indicating a refractive index of the second coupler. In the figure, the vertical axis and the horizontal axis represent the refractive index and the position on a cross section X-Y of the multi-mode waveguide path 202 (i.e., plane in a direction of the minor axis of the multi-mode waveguide path 202). This curve of refractive index provides a step-shaped curve axis-symmetrical about a centerline QL running along the major axis of the multi-mode waveguide path 202. Based on this distribution of the refractive index, the propagation direction of an optical signal can be controlled in the multi-mode waveguide path 202, thus obtaining a desired coupling ratio.
To obtain such a refractive index distribution of the multi-mode waveguide path 202, an impurity corresponding to each refractive index value must be diffused into the core layer of this multi-mode waveguide path 202. Therefore, problematically, the production efficiency of the second coupler is reduced below that of the first coupler 100 by as much as an increase of the number of processes for that impurity diffusion.
To guard against this, there has been a need for such an optical device that can provide a desired coupling ratio and, at the same time, can be manufactured easily.
To achieve the above-mentioned object, a multi-mode waveguide path according to the present invention is formed in the shape of an island. With this, the average value of the refractive index along a propagation direction of an optical signal in a region including this multi-mode waveguide path and a clad layer having therein this multi-mode waveguide path along this propagation direction is set as a desired value. For example, preferably, the multi-mode waveguide path consists of a plurality of waveguide paths (called convex-lens type waveguide path) having a cross sectional shape of a convex lens which is projected onto the main surface as viewed from the upper side the main surface side of the clad layer, which convex-lens-type waveguide paths are preferably arranged parallel to each other with an equivalent spacing therebetween along the propagation direction. Preferably, each of these convex-lens type waveguide paths is the same in shape and size. More preferably, the shape of these convex-lens type waveguide paths is both-side convex lens type.
According to such a configuration, the multi-mode waveguide path is formed island-shaped, thus making it possible to obtain a desired coupling ratio. Furthermore, this type of an optical device can be manufactured easily because it does not require the process of multi-step diffusing an impurity in contrast to the conventional construction.